1. Field of the Invention
The present invention relates to acoustic wave devices, including surface acoustic wave filters and boundary acoustic wave filters, and methods for manufacturing such acoustic wave devices. More specifically the present invention relates to an acoustic wave device including a piezoelectric substrate on which electrodes including IDT electrodes, wiring electrodes, and pad electrodes are arranged and a method for manufacturing such an acoustic wave device.
2. Description of the Related Art
Acoustic wave devices are widely used as bandpass filters and duplexers for mounting on radio-frequency (RF) circuits in communication equipment such as cellular phones. Known acoustic wave devices include surface acoustic wave devices, which use surface acoustic waves, and boundary acoustic wave devices, which use boundary acoustic waves.
An acoustic wave device includes a piezoelectric substrate and electrodes arranged on the piezoelectric substrate, including Inter-Digital Transducer (IDT) electrodes, wiring electrodes, and pad electrodes.
The Background Art section of Japanese Unexamined Patent Application Publication No. 2005-117151 discloses a surface acoustic wave device including wiring electrodes continuous with IDT electrodes and made of a layered metal film including a plurality of electrode films deposited in layers. According to the disclosure in Japanese Unexamined Patent Application Publication No. 2005-117151, this structure reduces the resistance of the wiring electrodes.
For a surface acoustic wave device of the invention disclosed in Japanese Unexamined Patent Application Publication No. 2005-117151, a first electrode film is initially arranged on a piezoelectric substrate to define electrode portions including IDT electrodes. A second electrode film is then arranged to overlap the first electrode film. The second electrode film is arranged in the regions where bus bars of the IDT electrodes, wiring electrodes, and pad electrodes are to be arranged. The second electrode film can be made thicker to reduce the wiring resistance. According to the disclosure in Japanese Unexamined Patent Application Publication No. 2005-117151, the second electrode film can be defined using an appropriate conductive material such as aluminum, copper, gold, or titanium.
An acoustic wave device such as the surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 2005-117151 might suffer migration in an electrode film defining wiring electrodes. Migration might cause a break in the wiring electrodes and a short circuit between wiring electrodes connected to different potentials due to whiskers and electrode scraps produced by migration.
Migration occurs when acoustic wave vibrations induced by IDT electrodes are applied to the wiring electrodes. In addition, migration probably occurs when heat due to passage of high-frequency power through the wiring electrodes is applied to the wiring electrodes. In particular, migration is more likely to occur in wiring electrodes made of aluminum or gold.